Background: Reverse Osmosis (RO) is an increasingly common method of desalination. A full scale water desalination system by membrane technology (RO) evaluated in a southern city (Jask) in Iran. Methods: First, data collection on water supply and network were performed. Analysis on most of the water quality parameters (Turbidity, pH, EC, Cl-, Na+, Alkalinity, Ca, Na, K, No3, No2, Fe, Mg, Mn, NH4, Po4, HCo3-, So42- etc.) was performed as standard methods. The membranes of the RO in the desalination system were Poly-Amid (CSM type). Results: The efficiency of the RO water desalination system was 94.16, 84.12, 92.00, and 96.17% respectively for Turbidity, Na+, Mg2+, So42-. The result shows a significant difference between influent and effluent water of the RO system. The produced water is in agreement with national standard of drinking water. Furthermore, water exited from the RO system for TDS, Ca+2, and Mg2+ was less than minimum limit of the guideline. Conclusion: The quality parameters of the water resource (EC, TDS, Cl-, Na+ etc.) were higher than Iranian drinking water standards. The RO technology modified the quality of the water parameters.